A hardware issue is always possible. I don’t recall anyone reporting exactly these symptoms that eventually were associated with hardware failure, but Mother Nature is never fair.
As a test, go to Settings → System → mCONNECT] → Autoplay and disable Autoplay.
Make sure that the signal plugs are firmly seated in the Line-In jacks. There is a switch that detects the plugs and if they are not firmly seated, CONNECT will claim that there is no physical connection. If this is the case, the input will disappear from the controller rather than simply muting.
Are there any issues with playing another source through that CONNECT?
Is there any sensitivity to physical shock? You should be able to ‘thump’ the unit with an open hand without any audible consequence -- assuming that this does not physically disturb the nearby turntable.
And there could be issues with the cable between CONNECT and the amplifier or the amplifier itself. Can you play another source, such as a CD player through the amplifier?
Network issues can cause audio drops, but these will not correlate with the Volume control.